Technical Field
The present disclosure relates to an electro-hydraulic brake system and a method of controlling the same. More particularly, it relates to an electro-hydraulic brake system, which generates required oil pressure with a motor according to braking performed by a driver and transmits the generated oil pressure to a wheel brake, and a method of controlling the same.
Background Art
Recently, a brake-by-wire technology adopting an electronic control system for control has been applied to a brake system of a vehicle, instead of an existing known general hydraulic control system.
The electronic control brake system generates required oil pressure by using an electric motor according to braking performed by a driver, and transmits the oil pressure generated by driving the motor to a wheel brake (wheel cylinder) of each vehicle wheel to generate braking force.
The electronic control brake system controlling oil pressure with an electronic actuator is generally referred to as an electro-hydraulic brake (EHB) system.
The EHB system easily and individually controls braking force generated by each wheel, and thus it is possible to easily implement a function, such as an electronic stability control (ESC) or an anti-lock brake system (ABS) function.
In a typical EHB system, a pump, which converts rotating force into linear force when a motor is driven to move a piston in front and rear directions, and forms oil pressure by the piston pressurizing braking oil within a chamber of a cylinder, has been widely used as the electronic actuator.
The EHB system detects pedal stroke according to a pedal operation of a driver through a sensor, and then controls braking force of each wheel with oil pressure generated by the pump by driving the motor.
The EHB system is provided with a pedal simulator enabling a driver to feel pedal pressure similar to that of a general hydraulic brake system.
Accordingly, when a driver steps on and pressurizes a pedal connected to a backup master cylinder, pressure of braking oil inside the backup master cylinder is increased, and oil pressure of the backup master cylinder is transferred to the pedal simulator through a pedal hydraulic line to generate a pedal feel.
When a driver steps on a brake pedal, a control unit calculates target oil pressure desired by the driver based on a driver pedal input value (braking input value), that is, a pedal stroke value, detected through a brake pedal sensor (pedal stroke sensor), and then makes a pump generate oil pressure by controlling driving of the motor according to the calculated target oil pressure, thereby transferring the oil pressure generated by the pump to each wheel cylinder and obtaining desired braking force.
FIGS. 1 to 3 illustrate an example of an EHB system in the related art.
As illustrated in FIG. 1, the EHB system in the related art includes a brake pedal 21 that is an input means for a brake will of a driver, a backup master cylinder 22 connected to the brake pedal, a pedal simulator 24 connected to the backup master cylinder 22 in order to provide an appropriate pedal feel, and an oil reservoir 10 for storing braking oil.
The backup master cylinder is connected to circuits for forming braking oil pressure to each wheel through cut valves 42a and 42b that are normally open valves, and provides oil pressure of the backup master cylinder as braking oil pressure of each wheel depending on a case. Further, the circuits are connected to a pressure generating unit 30 including a motor, and oil pressure formed through the pressure generating unit 30 is transferred to each of wheel clamping units 60a, 60b, 60c, and 60d, thereby providing braking force to each wheel.
Inlet valves 44a, 44b, 44c, and 44d are installed at distal ends of the circuits, respectively, so that oil supply lines 41a and 41b to each wheel are formed. Further, an oil return line 50 for returning oil to the oil reservoir when the ABS is operated is formed, and the oil return line is connected to the oil reservoir 10 by outlet valves 45a, 45b, 45c, and 45d. 
In the meantime, FIGS. 1 to 3 illustrate different operation states, respectively. Particularly, FIG. 1 illustrates a case where oil pressure is formed during a normal operation, FIG. 2 illustrates a case where an electronic control unit (ECU) has a failure, and FIG. 3 illustrates a case where oil leaks in some circuits.
As illustrated in FIG. 1, during a normal operation, all of the cut valves 42a and 42b are closed, so that a pedal feel is formed by the pedal simulator 24. In this case, oil pressure within each circuit is increased by the motor and braking oil pressure is provided to each wheel.
In the meantime, FIG. 2 illustrates a case where braking is performed in a situation, such as a situation in which the ECU has a failure, and when the ECU has a failure, it is impossible to control the motor, so that the cut valves 42a and 42b are opened, and a hydraulic line is formed so that the braking is performed by using an pedal effort according to step-on of the brake pedal by a driver.
FIG. 3 illustrates a state where oil leaks in a part of the circuits (a left circuit in FIG. 3). When oil leaks in a circuit as illustrated in FIG. 3, and the control unit detects the oil leakage, the control unit stops the operation of the pressure generating unit 30, and controls so that the hydraulic line is formed as illustrated in FIG. 2.
In the meantime, when oil leaks in the circuit and the control unit fails to detect the oil leakage, the pressure generating unit 30 is continuously operated according to a braking input of the brake pedal. Oil within each circuit returns to the oil reservoir 10 through the oil return line 50, and then oil of both circuits is joined again. Accordingly, when the pressure generating unit 30 is continuously operated, there is a problem in that an oil quantity required for braking is insufficient due to continuous oil leakage at a leak point.
An oil quantity for operating the motor is insufficient due to the continuous oil leakage, and the oil of the oil reservoir 10 is sucked through the oil return line 50, so that the pressure generating unit cannot operate the motor, thereby failing to sufficiently provide braking force. That is, even though the control unit detects that it is impossible to operate the motor, switches an operation mode into an oil leakage detection mode, and performs braking only with the backup master cylinder by opening the cut valves, an oil quantity capable of performing braking eventually becomes insufficient, so that it is impossible to perform braking. Further, braking force is formed only with a pedal effort of a driver, so that a problem in safety, such as an increase of a braking distance, is caused.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.